Abstract : Some basic concepts of Aerodynamics and Thermodynamics have previously been covered. These related to determination of fluid flow around various shapes and the resultant forces acting upon these shapes. Fluids previously studied were assumed to be incompressible. This assumption, among others, reduced the number of variables involved and allowed relatively simple solutions to previously complex sets of equations. Making assumptions to eliminate some variables is an everyday activity of the engineer, but care needs to be taken to ensure assumptions made to provide an idealized solution to a given physical system are still valid if the idealized solution is applied to a related, but different, physical system.